Field of the Invention
The invention pertains to the field of acoustic transducers. More particularly, the invention pertains to “free swinging” acoustical ribbon transducers with ribbon elements mechanically terminated only at their ends.
Description of Related Art
A ribbon audio transducer is a type of speaker or microphone, as shown in FIG. 11, in which the sound is generated by a thin strip of metal ribbon 111 which is suspended between two permanent magnets 110a and 110b. A small gap 115 between each of the magnets 110a and 110b and the edges of the ribbon 111 allows the ribbon 111 to vibrate. The ends of the ribbon 111 are held by mounts 113a and 113b which are electrically isolated from each other and electrically connected to the ends of the ribbon 111. An audio source 112 is connected to each of the mounts 113a and 113b by wires 114.
When the transducer is used as a speaker, an alternating current created by signal from the audio source 112 passes through the ribbon 111, and a varying magnetic field is created. The interaction of the static magnetic field between the permanent magnets 110a and 110b and the varying magnetic field around the ribbon 111 causes the ribbon 111 to vibrate in time to the alternating current, which causes the ribbon 111 to produce sound. If the transducer is used as a microphone, sound waves impinging on the ribbon 111 cause it to move, and as is known to the art, movement of a conductor through a static magnetic field causes an alternating current signal at the ends of the conductor.
This sort of ribbon type audio transducer, where the vibrating ribbon element 111 is mechanically terminated only at its ends or at its ends 113a 113b (and possibly at points along its length as in the case of a long ribbon divided up into shorter sections) is sometimes referred to as “free swinging” or “true ribbon” designs. These designs are in contrast to vibrating ribbon or planer magnetic elements that are terminated on all sides.
As shown in FIG. 7, ribbon audio transducer vibrating elements 71 are typically formed with corrugations 72 running perpendicular to the longitudinal axis of the ribbon element. This will be referred herein to as the “transverse direction”. These corrugations 72 produce a transverse stiffness to the vibrating element to resist flexures in this direction.
As shown in FIG. 6a, these transverse corrugations 62 which run in the transverse direction L1 provide a compliance or spring effect 63 along the length of the ribbon element. This compliance along the length of the ribbon element allows mild tensioning F of the ribbon 61 as it is mounted in its magnet assembly 60, and holds the ribbon element in the gap between rows of magnets that are part of said assembly. This compliance 63 along the ribbon element's length also allows the ribbon element to move freely in the lateral direction as is necessary in reproducing sound waves as in a loudspeaker or in response to sound waves as in a ribbon microphone. However the compliance of the ribbon element in the longitudinal direction that results from transverse corrugations is extremely high. This extreme compliance along the length of the ribbon element gives very little control of the “free swinging” ribbon element's movements in the lateral direction. This fact limits practical sized ribbon transducers with transverse corrugations to higher frequency operation where lateral movements are small.
The larger movements and increased lateral forces associated with lower frequency operation easily overwhelms transverse corrugated ribbon elements and can stress the ribbon material past its mechanical yield point resulting in an elongation of the ribbon element and a loss of the ribbon element's initially installed tension. Also this extreme compliance gives almost no control over standing wave activity at lower frequencies as the transversely corrugated ribbon easily submits to the forces producing this phenomenon resulting in limited power handling and low frequency response irregularities.
Another undesirable phenomenon associated with transversely corrugated ribbon elements is referred to as “twisting” where the ribbon undergoes a torsional movement along its longitudinal axis.
As a result of these performance issues, prior art ribbon designs that attempt to reproduce higher sound pressure levels and or lower frequencies have been undesirably large structures, increasing the ribbon's surface area so as to reduce the magnitude of the excursions associated with lower frequency operation. This fact has limited practical sized free swinging ribbon audio transducers with transverse corrugations to use only at the higher audio frequencies, typically above 1000 hertz or more.
To achieve reliable response below 1000 hertz, the free swinging designs have resorted to undesirably large designs ranging from approximately 2 to 7 feet in length. This spreads the drive forces of lower frequency operation out over a larger area thus reducing the peak to peak movements to a point where the ribbon elements lack of motional control is less problematic. This approach does not result in a practical sized loudspeaker as desired by most audio system users.
Walker, U.S. Pat. No. 4,550,228 shows a “Ribbon Speaker System” of the prior art. All but one of the Walker figures show ribbon elements with corrugations which are perpendicular to the longitudinal axis of the ribbon, as was common in the prior art. FIG. 7 of the Walker patent shows a ribbon element which has corrugations at “a variable angle relative to the vertical axis of the ribbon in order to provide a variable spring support in line with the acoustical drive and to provide mechanical crosswise stiffness”. This design does not use a “free swinging” ribbon element, as “ribbon 760 is attached to the inside edges of strips 780 and 781 means of pressure-sensitive-adhesive covered foam strips 772”.